This invention relates to a magnet assembly used in a sputtering chamber. In sputtering chambers, a magnet assembly is typically used to confine plasma gases in an area on the surface of a target. The transfer of momentum as ions strike the target causes atoms in the target to be ejected from the target and deposit on silicon wafers. The target may be an alloy such as tungsten silicide. The gas in the sputtering chamber during use is at a low pressure and gas ions such as argon gas ions are created to sputter metal from the target. These ions are confined by the magnetic field of a magnet assembly. The magnet assembly rotates behind the target at a rate of about 1 revolution per second so that the plasma sweeps across different areas of the target.
FIG. 1 is a top view of a prior art Anelva PMC magnet assembly which works with targets that are 280 millimeters in diameter. This magnet assembly uses a center magnet 2 which is oriented with the south pole on the top (directly behind the target), and ring magnets 4, which surround the center magnet 2. These ring magnets are oriented with the north on top. The whole magnet assembly 6 is symmetric about the two axes 3 and 5.
FIG. 2 is a cross-sectional view of a sputter target 11 after use with the Anelva PMC magnet assembly shown in FIG. 1. Notice that the target 11 has areas 10 of uneven erosion. This uneven erosion is caused by the plasma being contained over some areas of the target more than other areas of the target during the revolution of the magnet assembly. This uneven erosion is problematic because the metals used in the target are expensive, and any unused metal is wasted. Additionally, re-deposition of metal occurs in area 12. Re-deposition occurs when metal sputtered from a part of the target is re-deposited on other parts of the target. Eventually, as the target is heated up and cooled down through the sputtering process, metal from the re-deposited area 12 may flake off and fall onto the wafer, causing imperfections.
A prior art magnet assembly that has a somewhat more even erosion of the target is shown in the prior art FIGS. 3 (a) and (b) . FIG. 3 (a) is a cross-sectional view of the Anelva SPMC magnet assembly. This magnet assembly 21 uses a center magnet 20, thirty-two ring magnets 22 of which four are shown in FIG. 3 (a) , and twenty-three rubber side magnets 26. The magnetic field strengths of the ring magnet section 22, the center magnet section 20 and the side magnet section 26 as measured at the surface of these magnet sections are 1550 Gauss, 960 Gauss, and 500 Gauss respectively. The rubber side magnets 26 have the disadvantage that they may become demagnetized if the magnet assembly becomes heated during the sputtering process. Additionally, the Anelva SPMC magnet assembly 21 must be used with a 300 mm diameter target. If the Anelva SPMC magnet assembly is used with smaller targets, the shielding on the sides of the target may be damaged which would contaminate the silicon wafer. Additionally, the sputter target shielding must be re-designed to accommodate the larger diameter target.
FIG. 3 (b) is a top view of the Anelva SPMC magnet assembly 21. The ring section 22 is symmetric across axes 23 and 25. The center magnet section 20 is located asymmetrically with respect to axis 25 and the side magnet 26 is on one side of axis 25.
It is an objective of the present invention to have a magnet assembly that increases efficiency of the target erosion in a sputtering chamber. Another object of the present invention is to have a magnet assembly that erodes the target surface of 280 mm diameter targets in a more even fashion.